1. Field of the Invention
The present invention relates to a method for the manufacture, by a self-alignment technique, of a transistor having differentiated access regions. More specifically, this manufacturing method, which makes use of three masking layers, enables the self-alignment of the control electrode metallization on both access regions which have different characteristics in terms of dimensions and doping level.
This method can be applied to all transistors, of the bipolar or unipolar type, but it is especially adapted to field-effect transistors and, above all, to transistors that are made on III-V materials such as GaAs and work in microwave applications.
2. Description of the Prior Art
For, in this field, the very small submicronic dimensions of the gate length most often make it necessary to resort to special techniques that make up for the inadequate precision of the masking means.
Self-alignment techniques are presently much used owing to their influence on manufacturing cost and on the improvement of performance characteristics. However, in the case of microwave transistors on III-V materials, for which it is necessary to control the gate, source and drain resistance values, respectively R.sub.G, R.sub.S and R.sub.D, there is no practical and economical technology that can be used to differentiate the doping levels on the source side and drain side, beneath the gate and in the access zones.
For example, the technique known as the "side wall" technique, which uses a mask deposited on a side wall of a pattern, permits n' implantations but is incompatible with n" implantations of L.sub.DD for the fine gates (n' and n" correspond to two intermediate doping levels between light doping n and heavy doping n.sup.+, and L.sub.DD is an abbreviation commonly used to designate a lightly doped drain region).
Or again, there is the technique of the metallic mask made of nickel that rules out a new masking after an over-etching of the gate and is incompatible with an etching of the mask on the source side.
Finally, the technique of the dielectric mask made of SiO.sub.2 is limited by the selectivity of etching and is not compatible with differentiated n' and n" dopings.